This invention relates generally to magnetic resonance (MR) imaging techniques. In particular, the invention relates to MR imaging that is triggered and/or synchronized with patient sensors that detect physiological conditions, such as a heartbeat, blood pulse, or respiration of the patient being imaged.
Magnetic Resonance Imaging (MRI) is a widely accepted and commercially available technique for obtaining digitized visual images representing the internal structure of objects (such as the human body) having substantial populations of atomic nuclei that are susceptible to nuclear magnetic resonance (MR) phenomena. In MRI, nuclei in the body of a patient to be imaged are polarized by imposing a strong main magnetic field (B0) on the nuclei. The nuclei are excited by a radio frequency (RF) signal at characteristic MR (Lamor) frequencies. By spatially distributing localized magnetic fields surrounding the body and analyzing the resulting RF responses from the nuclei, a map or image of these nuclei responses as a function of their spatial location is generated and displayed. An image of the nuclei responses provides a non-invasive view of a patient""s internal organs and of other tissues.
As shown in FIG. 1, an MR imaging system typically includes a magnet 10 to impose the static magnetic field (B0), gradient coils 12 for imposing spatially distributed gradient magnetic fields (Gx, Gy, and Gz) having gradients along three respective orthogonal coordinates, and RF coils 14 and 16 to transmit and receive RF signals to and from selected nuclei of the body being imaged. The patient 18 lies on a patient table 20 such that a portion of the patient to be imaged is moved, in three-dimensions, into an xe2x80x9cimaging volumexe2x80x9d between the magnet and coils, which defines a field of view (FOV) of the MRI system. One or more sensors, such as electrocardiogram (EKG) sensor 21, may be positioned on the patient to monitor physiological conditions of the patient, such as the heartbeat.
The MRI system operator controls the system through a computer workstation 22 with a keyboard, screen and other operator input/output devices. The MRI system operator positions the patient within the imaging volume using a movable table 20, and may attach sensors 21 that monitor the patient during imaging.
Sensors 21 monitor the heartbeat, respiration, blood pulse and/or other physiological conditions of the patient. Signals generated by these sensors may be applied by the MRI system to trigger or synchronize MR imaging with the physiological condition(s) being monitored. For example, a heartbeat sensor generates a signal indicative of the patient""s heartbeat that is applied to trigger an MR imaging sequence synchronized with the beating heart. Synchronization of an MR image with a beating heart may improve the clarity of an still image of the beating heart or enable a real-time image of the heart. Similarly, sensors monitoring physiological conditions may be used to synchronize MR imaging with respiration blood pulses, and other conditions of the patient. The signal from the monitor may also be recorded synchronously with the image data and used for post-processing.
Sensors to monitor physiological conditions are well known. For example, EKG electrical sensors mounted on a patient""s skin detect electrical signals from the heart and generate signals indicative of the heartbeat. Fluid flow sensors mounted near the nose or mouth of a patient detect a patient""s breath and generate a respiration signal. Similarly, electromechanical sensors mounted on the chest or back of a patient detect changes in the shape of the abdomen to generate signals indicative of the respiration of the patient. Blood pulse can be detected by light sensors that detect light reflected from skin or by pressure sensors that detect pressure changes in an inflated bladder wrapped around the patient""s arm.
The placement of such sensors on the patient""s body can be critical. If the sensor is not placed optimally, then the triggering of MRI scans can be affected. For example, the analog signal generated by the sensor might be of insufficient magnitude to even pass through a detection threshold. Alternatively, the passage through a preset threshold may be incorrectly timed and/or unreliable. Thus, the sensor placement should be checked for correctness before expensive and time consuming actual MRI scans are conducted.
Providing an audio and/or visual signal feedback indicative of proper sensor placement on a patient prior to MR imaging can be difficult. MR imaging is extremely sensitive to stray electromagnetic emissions. Such emissions may be emitted by signal wires and circuits associated with sensors. To reduce interference due to extraneous emissions, wires and circuits within the MRI imaging room and especially near the patient are minimized. In particular, the wires within the MR imaging room are preferably limited to only those wires needed for the gradient coils and the RF coils. Other signal wires and circuits are generally precluded from the MR imaging room.
Conventional CRT oscilloscopes that are often used to provide a visual feedback of patient sensor placement cannot be used in the presence of the strong B0 magnetic field within an MRI gantry room. In addition, the additional wires and circuits associated with the oscilloscope may, if left in the gantry room, cause unnecessary interference with the MRI processxe2x80x94and thus must typically be located in an inconvenient place outside the gantry room. Similar problems of inconvenience occur if an MRI system monitor outside the gantry room is utilized to monitor sensor placement. Moreover, even if the oscilloscope or MRI system monitor were used, it would add to the cost of the MRI system. Still further, to view an auxiliary monitor, its visual display requires that an operator turn away from the MRI imaging screen to determine whether the sensor is properly placed and functioning. Thus, while in-room monitors have been developed for MRI systems, (See U.S. Pat. No. 5,184,074, issued to Kaufman et al.) they have various disadvantages such as cost, complexity and possible stray RF emissions.
In MRI it is sometimes desirable to trigger or synchronize the image acquisition sequence with a physiologically generated signal, such as the heartbeat, the blood pulse or the respiration. The typical MRI system provides an input where a digital ON-OFF signal is used to create within the machine such triggering or synchronization.
To generate this digital signal, an analog signal is first created by direct monitoring of the physiologic process of interest. Examples are EKG detection for the heartbeat, breath or abdomen shape for breathing, and pressure or reflected light for blood flow. The signal these processes create is continuous, and a trigger level or threshold has to be set to extract from them a desired ON-OFF signal.
The analog detection devices are attached to the body or otherwise interact with it, and because of patient-to-patient variability, operators need to locate them in certain ways, or need to test different locations to get a reliable signal. Thus, the signal needs to be monitored. This is typically done with an oscilloscope or by digitizing the continuous signal and displaying it on the MRI console monitor.
Oscilloscopes themselves need adjustment that is sometimes beyond the capabilities of the MRI operators, they add cost, and they do not work near the MRI magnets. The console display is reliable and simple, but requires that the operator move between the patient and the console, which is in a different room. This adds time to the setup process. An in-room monitor for real time display (U.S. Pat. No. 5,184,074) could be used to display the signal, but it adds cost.
There is a long-felt need for an economic device that provides sensor signal feedback, such as of an EKG heartbeat signal, to an MRI technician within the MRI gantry room. It is desirable that this device not add extra wires and circuits, especially within the MR imaging room.
This invention provides immediate real-time feedback to an operator of an MRI apparatus as to the adequacy of the placement of a physiologic sensor on a patient being prepared to undergo MRI. Basically, a special imaging sequencexe2x80x94one not actually used for producing any image data and perhaps consisting only of a single pulse of a single gradient coilxe2x80x94is gated (i.e., triggered) by the occurrence of a specific physiological phenomenon detected by a sensor placed on the patient. Since gradient coils typically produce a characteristic sharp noise when they are pulsed, the triggering of the special imaging sequence results in the production of an immediately noticeable and readily identifiable sound.
In practice, the physiological phenomenon-gated special imaging sequence mode is initiated just prior to placement of the sensor on the patient. The operator is then free to test different placements of the sensor. If the sensor is improperly placed, the special imaging sequence will not be triggered and the characteristic gradient coil noise will not be produced.
When the sensor is properly placed, however, and the special imaging sequence is properly triggered by the desired physiological process of the patient, the characteristic sound of the gradient coil noise will be audible and will follow the rhythm of the patient""s physiological process. In this manner, as the operator adjusts the sensor placement, the triggered imaging sequence sounds can be used as real-time audible feedback to guide the operator as to the adequacy and reliability of the selected physiological process or placement of the sensor. In addition, the method of the present invention may be easily implemented on most types of MRI equipment for little or no additional expense.
The present invention provides an essentially zero cost feedback signal for determining optimism positioning of sensors in an MRI gantry room. The invention does not add any wires or circuits to the MRI system, because it uses only an existing MRI gradient coil to generate an audible soundxe2x80x94triggered by the sensor as currently applied to a patient. Because it relies on an existing MRI system coil, the invention is inexpensive (especially in view of conventional oscilloscope feedback devices). Moreover, the invention provides an audible sound that was directly triggered by a heartbeat or other physiological condition. This immediately and clearly notifies the MRI technician performing the placement that the sensor is properly positioned and functioningxe2x80x94or, by its absence, that the sensor is not properly detecting the heartbeat or other physiological condition.
During this preparatory stage, the sensor signal is used by the MRI system to immediately trigger a special truncated test gated or triggered sequence, e.g., single pulse, applied to one of the gradient coils. This causes the gradient coil to vibrate, e.g., xe2x80x9cringxe2x80x9d, and generate a distinct sound that can be readily heard by the MRI operator in the gantry room. If the sensor is improperly placed or is malfunctioning, then it will not trigger the special test sequence and there will be no distinct sound from the gradient coil.
Obtaining good triggering signals from sensors applied to a patient can be problematic. To obtain a good signal, the sensors must be properly positioned on the patient. Positioning sensors on the patient to detect physiological conditions generally requires that the MRI system operator be able to see or hear a characteristic trigger signal generated by the sensor as feedback to the sensor positioning process. By hearing or seeing such a trigger signal indication from the sensor, the operator can determine when the sensor is properly placed on the patient. For example, EKG sensors must be positioned on the patient so as to properly detect electrical signals from the heartbeat. To determine whether the sensors are properly positioned, the operator places the sensors on the patient""s chest in the expected vicinity of where the sensors can detect a heartbeat signal. The sensors may be connected to an EKG monitor so that the operator can then see or hear the heartbeat signal being detected by the sensors. If the signal is not clear and/or strong, the operator can move the sensors around on the chest until the detected EKG signal is strong and clear. Similarly, before a sensor-triggered MRI scan is conducted, the technician placing the sensor(s) on the patient in the MRI gantry room needs to have feedback to be sure that the sensor(s) is(are) properly placed. With this invention such feedback is provided by audible signals conveniently and immediately in the gantry room and at essentially zero added cost. Thus, the technician does not need to go back and forth between the patient and a monitor outside the gantry room.
The acoustic gated monitor is only used during patient setup. The operator has to be in the room for setting up the sensor system, e.g., EKG, finger plethismograph, pressure cuff, breath flow, whatever. This requires adjusting for picking up a physiologic signal. How does the operator know if the placement is correct? Having an oscilloscope in the room is inconvenient: if it is not shielded it has to be brought in for the setting up and taken out before imaging starts, and the magnetic field affects it. Going out to look at the console monitor is very inconvenient looking at a scope that is a distance from the magnet while playing with the patient sensor in or near the magnet is inconvenient. So the operator turns on one (or more) gradient sequence (without RF to cause interference) and listens as sensor adjustments are made. The operator positions the sensor on the patient. When the sensor is correctly positioned, the gated pulses to the coils generate sounds heard by the operator. These sounds inform the operator that the sensor is properly positioned. Once the proper positioning of the sensors is confirmed by the sounds from the coil, the operator leaves the gantry room and starts a conventional imaging sequence.